The present invention relates to microelectromechanical devices and associated fabrication methods and, more particularly, to an encapsulation process for microelectromechanical structures and associated microelectromechanical devices.
Microelectromechanical structures (MEMS) and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created, including microgears, micromotors, and other micromachined devices that are capable of motion or applying mechanical force. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are utilized, optical applications which include MEMS light valves and shutters, and electrical applications which include MEMS relays.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired mechanical motion within these microstructures. For example, electrostatic actuators have been used to actuate MEMS devices. See, for example, U.S. patent application Ser. No. 09/320,891, assigned to MCNC, also the assignee of the present invention, which describes MEMS devices having electrostatic microactuators, the contents of which are incorporated herein by reference. In addition, controlled thermal expansion of an actuator or other MEMS component is another example of a technique for providing the necessary force to cause the desired mechanical motion within MEMS devices. See, for example, U. S. Pat. No. 5,909,078 and U.S. patent application Ser. Nos. 08/936,598; and 08/965,277, assigned to MCNC, also the assignee of the present invention, which describe MEMS devices having thermally actuated microactuators, the contents of which are incorporated herein by reference.
Once a MEMS device has been fabricated, the entire device must undergo subsequent packaging steps to process the MEMS device into a usable form. These packaging steps may include, for instance, wafer dicing, assembly, wire bonding, and encapsulation processes. A typical MEMS device is unlikely to survive these packaging steps due to the extensive manipulation of the device during the individual processes. Since the actuators used in MEMS devices incorporate mechanical motion to achieve the desired function of the particular MEMS device and since a MEMS device may include additional mechanically sensitive components, MEMS devices generally present a particularly challenging packaging problem.
Conventional integrated circuit encapsulation packaging is typically a conformal surface coating. However, conformal coatings are not particularly suited to packaging a MEMS device since it difficult to provide the necessary clearances about the mechanically sensitive components of the MEMS device.
Another approach to packaging MEMS devices has been to fabricate a separate xe2x80x9clidxe2x80x9d structure which is then bonded to the MEMS die prior to packaging. However, a disadvantage of the separate lid approach is that, when the lid is bonded to the die at the wafer level, the entire MEMS die is covered, thereby preventing physical or electrical access to the MEMS die in subsequent packaging processes.
Thus, there exists a need for an encapsulation process for a MEMS device which is compatible with and capable of protecting the mechanically sensitive components of a MEMS device during subsequent packaging steps. Preferably, the encapsulation process for a MEMS device utilizes conventional semiconductor fabrication techniques and equipment such that special measures are not required. Further, the encapsulation process is desirably capable of selectively encapsulating portions of the MEMS device while leaving other portions free of the encapsulant which, for example, do not require encapsulation or which must be externally accessible in subsequent packaging processes. In addition, the encapsulation process for a MEMS device is preferably cost-efficient and allows conventional low-cost packaging techniques to be used following encapsulation of the MEMS device.
The above and other needs are met by the present invention which, in one embodiment, provides a method of encapsulating microelectromechanical (MEMS) structures formed on a substrate prior to packaging thereof. First, a sacrificial material is deposited over the substrate to cover at least a portion of the MEMS structure. An encapsulation material is then deposited over the sacrificial material such that the encapsulation material covers at least a portion of the sacrificial material over the MEMS structure. The sacrificial material is subsequently removed such that the encapsulation material forms a shell spaced apart from and covering the MEMS structure and permits the intended operation of the MEMS structure.
According to another advantageous embodiment of the present invention, the step of depositing a sacrificial material may further comprise depositing a removable photoresist on the substrate to cover at least a portion of the MEMS structure where, in some cases, the photoresist completely covers the MEMS structure. Further, a method of encapsulating MEMS structures may include the step of forming a pattern associated with the sacrificial material to selectively define regions of the MEMS structure that are covered during subsequent operations. The sacrificial material may also define regions of the MEMS structure that are to be protected by the encapsulation material.
Advantageous embodiments of the present invention also include the step of defining at least one opening in the sacrificial material following deposition thereof for exposing a portion of the substrate, the exposed portion of the substrate comprising, for example, an anchor point. In addition, the step of depositing an encapsulation material may further comprise depositing an encapsulation material over the sacrificial material such that the sacrificial material engages the MEMS substrate at an exposed portion thereof, wherein the encapsulation material may comprise, for example, a photoimagable epoxy having a sufficient thickness to form an encapsulating shell about the MEMS structure. After the encapsulation material has been deposited on the sacrificial layer, at least one opening in the encapsulation material is then defined to expose a portion of the sacrificial material. Some embodiments of a method for encapsulating MEMS structures include the step of forming a pattern associated with the encapsulation material to selectively define regions of the MEMS structure that are covered by the encapsulation material during subsequent operations. Generally, the step of depositing an encapsulation material comprises depositing an encapsulation material over the sacrificial material after the sacrificial material has been patterned and portions thereof removed.
In some instances, the step of removing the sacrificial material may further comprise removing the sacrificial material such that the encapsulation material forms a shell spaced apart from and covering at least a portion of the MEMS structure such that the intended operation of the MEMS structure is permitted, wherein the sacrificial material is generally removed from areas of the MEMS structure that are not desired to be in contact with the encapsulation material. In addition, the step of removing the sacrificial material may further comprise removing the sacrificial material from portions of the MEMS structure that are to be externally accessible following the deposition of the encapsulation material. Prior to the step of removing the sacrificial material, embodiments of a method of encapsulating MEMS structures include the step of selectively insolubilizing the encapsulation material to make it more durable and impervious to subsequent processing steps. Thus, a method of encapsulating MEMS structures formed on a substrate prior to packaging thereof generally comprises the step of depositing an encapsulation material over the MEMS structure such that the encapsulation material forms a shell spaced apart from and covering the MEMS structure, thereby permitting the intended operation of the MEMS structure.
Another advantageous aspect of the present invention comprises an encapsulated MEMS structure having a microelectronic substrate with at least one MEMS structure formed thereon and an encapsulating cover connected to the substrate and forming a shell over at least a portion of the MEMS structure such that the intended operation of the MEMS structure is permitted. The substrate may further define anchor points for the cover where the anchor points are fixed in location and the encapsulating cover generally comprises vertical supports and generally horizontal segments. In some instances, the encapsulating cover comprises a shell having supports connected to the microelectronic substrate at the anchor points. Additionally, the encapsulating cover may define access zones where access may be gained to the MEMS structure underneath the shell. The encapsulating cover may be formed of an epoxy material and, more specifically, for example, a photolithographically patternable material that is microelectromechanically fabricated.
A further aspect of the present invention comprises an intermediate MEMS structure having a microelectronic substrate with at least one MEMS structure formed thereon, a sacrificial layer over the substrate that covers at least a portion of the MEMS structure, and an encapsulating layer over the sacrificial layer that covers at least a portion of the sacrificial layer over the MEMS structure and is connected to the microelectronic substrate. Preferably, the microelectronic substrate defines fixed anchor points for the encapsulating layer, wherein the encapsulating layer comprises a shell having supports connected to the microelectronic substrate at the anchor points. The encapsulating layer is preferably comprised of an epoxy material such as, for example, a photolithographically patternable material that is microelectromechanically fabricated.
Still another advantageous aspect of the present invention comprises a system having an operational device that incorporates a MEMS structure, an input to the MEMS structure, a MEMS structure having an encapsulating cover forming a shell over at least a portion of the MEMS structure and spaced apart therefrom such that the intended operation of the MEMS structure as permitted, and an output from the encapsulated MEMS structure. Preferably, the input originates at least in part from the operational device and the output is provided at least in part to the operational device. Such an operational device may comprise, for example, a high voltage switch, a gyroscope, and an inertia sensor. More generally, the operational device may be capable of, for instance, directing light and controlling airflow.
Thus, embodiments of the present invention provide an encapsulation process for a MEMS device which is compatible with, and capable of protecting, the mechanically sensitive components of a MEMS device during subsequent packaging steps. As described herein, the encapsulation process for a MEMS device according to the present invention utilizes conventional semiconductor fabrication techniques and equipment and is capable of selectively encapsulating portions of the MEMS device while leaving other portions free of the encapsulant which, for example, do not require encapsulation or which must be externally accessible in subsequent packaging processes. Further, the encapsulation process according to the present invention is cost-efficient and allows conventional low cost packaging techniques to be used following encapsulation of the MEMS device.